Electron microscope vacuum system



Dec. 16, 1941. F. KRAusE ELECTRON MIQROSCOPE VACUUM SYSTEM Filed Mayv 15, 1940 'luana 'funn'.

Patented Dec. 16, 1941 j VELEcfiRoN MicRoscoPE-v-ACUUM SYSTEM M Friedrich Krause, Berlin, Germany Animation May 13, 1940, seriaiNo.

In Germany May 8, 1,939

4 claims (c1. 25er-iin' The present invention relates to electron microscopes and more vparticularly pertains to means for maintainingdiiierent pressures there-- 1n.

Y It is essential for satisfactory sha-rp depicting and high resolving of the images inV an. electron microscope that the `course of the electrons in the apparatus be not influenced in additiento the diffraction in the object or the deflection in the coils.k It .is known Vthat 'electrons -are influenced by layers of air similarly to light `rays by opal glass.

It is therefore of importance to produce a very high 'vacuum below IQ-'3 millimeters of mercury in the electron microscope. This applies more particularly to points which are of essential importance for the formation of images as, for instance, directly below the object, at the lenses and at similar points.

However, it may be necessary to employ higher pressure at certain points of the apparatus. The use of cold cathodes as light ray source of an electron microscope may be indicated as an example. Pressures of about -2 millimeters of mercury are necessary in this case but such pressures in the depicting space of the electron microscope gives rise to blurred images. Moreover, the presence of gas particles at other points of the apparatus may be of great importance. For example, it is frequently necessary when examining organic substances such as cellular films and the like to employ a lower vacuum of about 10-1 to 10-2 millimeters of mercury as is otherwise necessary for a sharp depicting possibility in the electron microscope. In the case of very high vacuum (below 10-3 millimeters of mercury) the dried brittle organic films are charged with electron rays during the examination. This may cause a distortion of the electron image by space charges and lead to a rupture of the films. The secondary heat produced during the passage of the electrons through the material can not be carried away which eventually leads to combustion. For this reason, it is advisable to carry away a large number of air particles at such points in the electron microscope where charging or heating is to be avoided. This may be eiected by admitting air at these points through an air inlet valve.

In order to prevent the electron images from being affected by the aforesaid conditions or circumstances, there are provided according to the invention separate pump connecting pipes at the important image points which provide for a continuous suction of the residual gases or of the coolinggases employed. The arrangement =of separate pump Aconnecting pipes atV variousy points enables varying vacuum to be provided in the Yelectron n'lic-roscopeI atindividualpoints `according to requirerrient.'r Such: important irri-- age pointsare situated for instance just below the anode diaphragm lenses. The pipes mayY be connectedgseparately' to a separate pump Ager-several pipes may.beconz nectedto a common pumping plant. l, A constructional form of the inver-ubica.isv shown lby way of example in the-accompanying drawing( The electron-microscope civ-fis..,providedy with a cold-cathode b iorvvoltages-oi about,89,-i kilo-volts and subjected to a pressure of about 10'-2 millimeters of mercury. An air inlet valve (not shown) is arranged adjacent the cathode of the electron microscope. In order that the vacuum of 10-2 millimeters of mercury, which is not suitable for a sharp depicting of the image,

may not pass into the lower part of the electron microscope, a throttle diaphragm d is provided in such a manner that it divides the high vacuum space of the electron microscope into an upper space belonging to the discharge tube and into a lower space belonging to the object and the lenses. A pump connecting pipe e', connected to a pumping plant (not shown) is provided just below the anode diaphragm c.

Separate pump connecting pipes are provided at other important image points, in this case just below the lenses a pipe e2 is provided below the objective lens f and a pipe e3 is arranged below the projection lens g. The `object (not shown) is arranged just below the objective lens f.

This arrangement of different pump connecting pipes at the important image points of the electron microscope renders' it possible to provide the desired pressure of about 10-3 millimeters of mercury in the discharge tube in the vicinity of the cathode b and to provide substantially lower pressures of about 10-4 millimeters of mercury in the lower space of the electron microscope.

In the constructional form shown in the drawthe pressure variations as for instance gas .out-

breaks or escapes or the like.

The housing h may be of great importance if the hollow space in the electron microscope is small and the apparatus isy e formed of metal.

and in the vicinity of; thei A housing h extending from said tube intermediate the diaphragm and the anode for adjusting the pressure within the tube adjacent the cathode, objective and projecting lens means associated with the tube, and pipes communicating with the tube adjacent the lens means for adjusting the pressure in the tube adjacent an object to be analyzed.

2. In an electron microscope, a sealed tube, a

cathode within said tube, an apertured anode Within said tube, a diaphragm having a throttling opening therein spaced from said anode, a pipe extending from said tube intermediate the diaphragm and the anode for adjusting thepressure Within the tube adjacent the cathode, objective and projecting lens means associated with the tube, conduits extending from said tube adjacent the lens means, a pressure equalizing housing connected to said conduits, and a pipe extending from said housing for adjusting the pressure therein.

3. In an electron microscope, a sealed tube, a cold cathode Within an end of said tube, an apertured anode within said tube, a neutral diaphragm4 substantially closing said end of the sealed tube and spaced from the anode, said diaphragm having a throttling opening therein, a pipe extended from said tube intermediate the diaphragm and the anode for adjusting the pressure Within the tube adjacent the cathode, objective and projecting lens means associating with said tube and spaced therealong for arranging an object to be examined therebetweenand pipes communicating with the sealed tubeadjacent the lens means for adjusting the pressure within the tube adjacent the object to be analyzed to value different from the pressure adjacent the cathode.

4. In an electron microscope, a sealed tube, a cold cathode withinan end of said tube, an apertured anode within said tube, a neutral diaphragm substantially closing said end of the sealed tube and spaced from the anode, said diaphragm having a throttling opening therein, a pipe extended from said tube intermediate the diaphragm and the anode for adjusting the pressure Within the tube adjacent the cathode, objective and projecting lens means associating with said tube and spaced therealong for arranging an object to be examined-therebetween, and conduits extending Vfrom said tube adjacent the lens means, a pressure equalizing housing connected to vsaid conduits, and a pipe extending from said housing for adjusting the pressure Within the tube adjacentthe lens means to a value dierent from the pressure within the tube adjacent the cathode.

` FRIEDRICH KRAUsE. 

